The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in magnetic resonance angiography (MRA).
In MRI, a uniform magnetic field Bo is applied to an imaged object along the z-axis of a Cartesian coordinate system fixed with respect to the imaged object. The effect of the magnetic field Bo is to align the object""s nuclear spins along the z-axis.
In response to a radio frequency (RF) excitation signal of the proper frequency oriented within the x-y plane, the nuclei precess about the z-axis at their Larmor frequencies according to the following equation:
xcfx89=xcex3B0xe2x80x83xe2x80x83(1)
where xcfx89 is the Larmor frequency, and xcex3 is the gyromagnetic ratio which is a constant and a property of the particular nuclei. The component of the nuclear spins aligned with the x-y plane is termed the transverse magnetization.
The rate of decay of the transverse magnetization differs for different tissues and hence may be used to distinguish among tissue in an MRI image. Hydrogen, and in particular the nucleus (protons) because of its relative abundance in biological tissue and the properties of its nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio g for protons is 4.26 kHz/gauss and therefore in a 1.5 Tesla polarizing magnetic field Bo, the resonant or Larmor frequency of protons is approximately 63.9 MHz.
In a typical imaging sequence for an axial slice, the frequency of the RF excitation signal is centered at the Larmor frequency of the protons and applied to the imaged object at the same time as a magnetic field gradient Gz is applied. The gradient field Gz causes only the nuclei, in a slice with a limited width through the object along an x-y plane, to be excited into resonance.
After the excitation of the nuclei in this slice, magnetic field gradients are applied along the x and y axes. The gradient along the x-axis, Gx, causes the nuclei to precess at different frequencies depending on their position along the x-axis, that is, Gx spatially encodes the precessing nuclei by frequency. The y axis gradient, Gy, is incremented through a series of values and encodes the y position into the rate of change of phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by the RF coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well known reconstruction techniques. An overview of NMR image reconstruction is contained in the book xe2x80x9cMagnetic Resonance Imaging, Principles and Applicationsxe2x80x9d by D. N. Kean and M. A. Smith.
The delay between the RF excitation and the recording of the NMR data may be used to detect and measure the flow of blood in blood vessels and thereby to detect obstructions and to distinguish the blood vessels from stationary tissue as demarcated by the flowing blood.
Such flow measurement may be made most simply by selectively exciting the spins in a given location and measuring the transverse magnetization of the spins at a downstream location a short while later. Examples of this xe2x80x9ctime of flightxe2x80x9d technique are described in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793 and 4,777,957, hereby incorporated by reference. A variation of this technique notes the change of transverse magnetization in the region excited by the RF pulse. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,474; and 4,516,582; also incorporated by reference. A third technique measures flow by making use of the fact that spins moving in a gradient magnetic field experience a phase shift. This technique is described in U.S. Pat. Nos. 4,609,872 and 5,281,916, hereby incorporated by reference.
The quality of the image produced by MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, it is known to use an independent RF receiving coil placed in close proximity to the region of interest of the imaged object in order to improve the strength of this received signal. Such coils are termed xe2x80x9clocal coilsxe2x80x9d or xe2x80x9csurface coilsxe2x80x9d. The smaller area of the local coil permits it to accurately focus on NMR signals from the region of interest. Further, the RF energy of the field of such a local coil is concentrated in a smaller volume giving rise to improved signal-to-noise ratio in the acquired NMR signal.
The signal-to-noise ratio of the NMR signal may be further increased by employing a coil that is sensitive to RF energy along both of a pair of mutually perpendicular axes. This technique is generally known as quadrature detection and the signals collected are termed quadrature signals.
The outputs of the quadrature coil pairs are combined so as to increase the strength of the received signal according to the simple sum of the output signals corrected for phase shift from the coils. The strength of the uncorrelated noise component of these signals, however, will increase only according to the square root of the sum of the squares of the noise components. As a result, the net signal-to-noise ratio of the combined quadrature signals increases by approximately {square root over (2)} over the signal-to-noise ratio of the individual signal.
The quadrature orientation of the two coils introduces a 90xc2x0 phase difference between the NMR signals detected by these coils. Therefore, combining the outputs from the two quadrature coils, to achieve the above described signal-to-noise ratio improvement, requires that one signal be shifted to have the same phase as the other signal so that the amplitudes of the signals simply add.
Such phase shifting and combining is typically accomplished by means of a hybrid network. Hybrid networks are four-port networks known in the art and having the property that when the four ports are properly terminated, energy input to two of the ports, with the proper relative phase angles, will be combined at one of the remaining two ports. The antenna coils are attached to two of the ports and the output lead is attached to a third port and produces the sum of the signals from the antenna coils, one being shifted so that they add in-phase. The remaining uncommitted port is connected to a termination resistor.
As used herein, the term quadrature coil and quadrature signal, will refer to the detecting of the NMR signal along multiple axes and combining the signals so collected, with the appropriate phase shifts to produce a signal of improved signal-to-noise ratio.
One method of constructing a local coil is the xe2x80x9cbird cagexe2x80x9d construction in which two conductive loops are spaced apart along a common longitudinal axis and interconnected by a series of regularly spaced longitudinal connectors. The impedance of the loops and of the longitudinal conductors is adjusted so that the coil may be excited into resonance by a rotating transverse magnetic field at the Larmor frequency. A quadrature signal may be obtained by monitoring the current through two longitudinal conductors spaced at 90xc2x0 around the periphery of the loops. Such coils are described in detail in U.S. Pat. Nos. 4,680,548, 4,692,705, 4,694,255 and 4,799,016.
The use of volumetric local coils of conventional bird cage or other quadrature design may be undesirably constraining to the patient who must be surrounded by the relatively small volume of the coil. The use of a conventional volumetric coil for angiographic imaging of the lower extremities would require threading the patient""s feet through a relatively long tubular structurexe2x80x94a procedure that may be difficult or impossible for many patients.
For this reason it is known to produce an angiographic coil having flexible side panels supporting the coils, the side panels being folded around the supine patient after the patient is centered on the coil. See U.S. Pat. No. 5,594,337 assigned to the same assignee as the present invention and hereby incorporated by reference.
The present invention provides an angiographic coil that better conforms to patients with different body habitus and that provide improved coil orientation throughout its range of adjustments. Generally, side panels of the coil are broken into a number of flexible leaves which may be individually wrapped about the patient. The leaves allow taper in the overall coil form, and an ability to adjust this taper while maintaining the individual coils parallel to the longitudinal axis of the coil.
Specifically, the present invention provides an MRI coil for imaging the lower trunk and legs of a patient. The coil includes a base sized to fit against a table of an MRI machine, extending along a longitudinal axis and having transversely opposed left and right sides. A plurality of flexible coil leaves extend transversely from the left and right sides in opposed pairs, the pairs having progressively decreasing combined lengths from a first longitudinal end of the base to a second longitudinal end of the base. The pairs may be wrapped around a supine patient positioned on the base to form enveloping coils of progressively decreasing diameters.
Thus it is one object of the invention to provide a tapered coil that maintains proper coil orientation of each coil element. This orientation is maintained by breaking the flexible panels into leaves each which may be adjusted individually while maintaining parallel orientation.
First ends of the flexible coil leaves, opposed to second ends attached to the base, may hold adjustable fasteners attaching the first ends of each pair together when they envelope a patient.
Thus it is another object of the invention to provide a tapered coil whose effective taper may be adjusted without disrupting the proper orientation of the coils in the adjustment process. A tipping of the coils can cause undesired coupling between coils.
The flexible coil leaves may include conductors surrounding a coil area. Adjacent flexible coil leaves are attached to the left and right sides of the base so that their coil areas overlap along the longitudinal direction.
Thus it is another object of the invention to provide a multi-leaf, flexible coil having electrical isolation of adjacent coils. overlapping the coil areas reduces coupling between the coils as is necessary for high quality imaging.
A first and second flexible coil leaf may incorporate a flexible support attached at one end to a transverse side of the base to support the conductors. An inner resilient pad may be attached to a first face of the flexible support facing the patient when the coil leaf envelops the patient. The flexible supports of the first and second flexible coil leaf overlap at corresponding first and second transverse edges and the inner resilient pad of the first flexible coil leaf is inset from the first transverse edge and the resilient pad of the second conductor extends to abut the inner resilient pad of the first flexible coil providing a substantially continuous inner padded layer.
Thus it is another object of the invention to allow overlapping flexible coil leaves providing the above benefits and with continuous padding to the patient.
The MRI coil may include a foot coil attached to the second longitudinal end of the base, the foot coil having a first and second transversely extending flexible coil leaf having a smallest combined length of all leaves wherein at least one of the flexible coil leaves has an aperture allowing exit of the patient""s toes when the flexible coil leaves are wrapped around a patient""s foot.
Thus it is another object of the invention to provide a peripheral angiographic coil that may conform closely to the patient""s body despite the need to accommodate the patient""s toes. An aperture in the foot coil allows its leaves to be closely wrapped about the patient""s ankle.
Other objects and advantages besides those discussed above will be apparent to those skilled in the art from the description of the preferred embodiment of the invention which follows. Thus, in the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate one example of the invention. Such example, however, is not exhaustive of the various alternative forms of the invention. Therefore, reference should be made to the claims which follow the description for determining the full scope of the invention.